Methods for constructing filament wound vessels, tanks and containers are well known in the prior art. When composite pressure vessels, such as rocket motor cases or pressurized tanks, are fabricated on a mandrel so that there are end openings in the vessel which are smaller than the overall diameter of the vessel, there is the problem of removing the mandrel from the vessel. Typically, it is not advisable to cut the vessel from the mandrel and then join and bond the vessel pieces together to form the desired pressure vessel. This is because the joint in the vessel creates an inherent weakness which may fail or fracture when the vessel is subjected to pressure or stress. It is more desirable to remove the mandrel through the small openings in the vessel.
Heretofore, the problem of removing the mandrel from the composite vessel usually has led to one of three solutions. The first is to use a sacrificial mandrel which is destroyed upon removal. Such one-use mandrels can be expensive and vessel fabrication is time consuming when the number of units to be fabricated is considerable. Another solution is to use a collapsible mandrel that is disassembled and removed from the vessel, later to be reassembled and used again. Such mandrels are known as segmented mandrels. Use of segmented mandrels require time consuming assembly and disassembly. Additionally, the fabrication costs for a segmented mandrel that can consistently meet specified dimensional tolerances upon repeated disassembly and reassembly are, in many cases, prohibitively expensive. Still another solution is to use an inflatable mandrel which has the ability to eliminate the repetitive mandrel fabrication costs in the case of sacrificial mandrels, and the assembly/disassembly costs in the case of conventional segmented mandrels.
Inflatable mandrel development for composites started in the United States approximately twenty years ago. Initially, slosh coating an elastomeric material was used, followed by stitching of rubberized gore patterns to fit the desired shape.
One current method for fabricating an inflatable mandrel was developed, using uncured silicone rubber sheet stock, cut and formed to the desired mold shape. Initially, a wooden male pattern is made to match the external configuration of the inflatable mandrel to be fabricated. Next, two mating female plaster patterns are made. A male plastic laminating mold is cast in the female plaster molds, coated with a hardener, and cured. The plastic laminating mold is release coated with a mold release and sheets of uncured silicone rubber sheet stock are cut and fitted into each half of the mold. Additional layers of silicone rubber sheet stock reinforced with fabric, are fitted into the plastic laminated mold for tear resistance. An end plate is prefit into the uncured silicone rubber mandrel being fabricated. The two halves of the plastic laminated mold are separated, and a layer of silicone adhesive rubber is applied. The plastic laminated mold halves are carefully aligned and bolted together. The closed plastic laminated mold is pressurized to check for leaks. An external envelope vacuum bag is installed over the plastic laminated mold and checked for leaks before placing it in an autoclave. When placed in the autoclave, the uncured silicone rubber inflatable mandrel is purged of air and cured under pressure at temperatures of about 350.degree. F. A coat of mold release is applied to the inflatable mandrel after it is removed from the plastic laminated mold. The mandrel is then post cured at 400.degree. for 4 hours.
Another known method for fabricating an inflatable mandrel is disclosed in U.S. Pat. No. 4,684,423. Under this method a rigid mandrel is prepared and supported on a rotatable axle. One or more layers of strips of rubber are applied longitudinally on the mandrel to form an enclosure. Coats of rubber solution are applied over the layers, and a layer of fibers is wound over at least one of the layers. The rubber layers are then cured. The layers are cut into two parts, removed from the rigid mandrel, and rejoined by using reinforcing straps along the seam of the joined parts. This collapsible mandrel may be used to fabricate either filament wound tanks or other collapsible mandrels, neither of which would have a seal or joint.
However, no known inflatable mandrel is suitable for fabrication of a pressure vessel and is removable through a small opening (e.g., an opening of about 40% of the diameter of the vessel) in the vessel. Consequently, it would be an advance in the art of inflatable mandrel fabrication to provide a method for fabricating a collapsible inflatable mandrel that is streamlined to reduce fabrication costs and time and is capable of creating a seamless vessel.